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GE 6477 DISCONTINUOUS ROCK4. Jointing Genesis

Dr. Norbert H. Maerz

Missouri University of Science and Technology

(573) 341-6714

norbert@mst.edu

Instructional Objectives

1. Critique the proposed mechanics for the formation of the special cases of jointing.

2. Explain the principles of fracture mechanics concepts and why they do not do adequately explain the formation of ordinary joints.

3. Explain Griffith crack theory, and it implications in fracture propagation.

4. Critique the theory that hydrofracturing is a process that facilitates the creation of ordinary joints.

5. Interpret the consequence of the orthogonal nature of jointing with the propose mechanisms of jointing.

6. Rationalize the observations of Scheidegger with prevalent theories of joint formation.

Jointing Genesis

• Exceptions: Columnar Basalts, Sheeting/Exfoliation Joints, Layered Rock, Faults.

• Fracture Mechanics

• Typical “Ordinary” Jointing

• Hydraulic Fracturing

When

• Relatively fresh partially consolidated sediment to millions of years old

• Healing of joints over time - thermal metamorphism, pressure solution.

• Joints are a stress release mechanism, resulting in a complex redistribution of the stress field.

• Complex history - many episodes!

Special Case:Columnar Basalt

From the collection of J.A. Franklin.

Shrinkage Mechanism

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Monroe and Wicander

Plumose Structure

Special Case: Sheeting/Exfoliation Joints

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Sheeting Genesis Theories

Thermally generated compressive stresses

Fracture and displacement

Pop-ups

From the collection of J.A. Franklin.

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Special Case: Layered RockFlexural tension cracks

Special Case: Faulting

From the collection of J.A. Franklin.

Special Case, Faulting

From the collection of J.A. Franklin.

Faulting modes

• Normal Fault

• Reverse Fault

• Translational Fault (strike-slip)

Monroe and Wicander

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Monroe and Wicander

Special Case: Stylolites

Stylolites

http://www.gly.uga.edu/railsback/BS/BS-TSC.html

Formation of Stylolites

Fracture Mechanics Modes of Fracture Propagation

• Mode I - Tension

• Mode II - Shearing parallel to fracture front

• Mode III - Shearing perpendicular to fracture front

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Tensile Failure of Rock: Brazilian Test

Simple Direct Tensile Test

• Location of fracture plane cannot be pre-determined

• Most likely starts in a flaw or microcrack

Compressive (Shear) Failure of Rock

23

21 cossin n

cos22

3131

n

cossin)( 31

2sin2

)( 31

Mohr Circle

Idealized Multiple Test Mohr Circle

tannc

Frictionless (Cohesive) Material (Clay)

c

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Frictional Material (Sand)

tann

Actual Envelope for Rock

)( functionempirical

Influence of Pore Pressure

' '

Pre-existing Crack

Griffith Crack Theory

• Brittle materials material have microscopic flaws at random orientations.

• Fractures in these materials will make use of any opportunely oriented “Griffith” cracks.

Ideal strength of a brittle solid

• T=E/10 (ideal)

• example, Berea sandstone– E = 19 Gpa (modulus of elasticity)

– T = 1 Mpa (tensile strength)

– E/T = 19000 not 10!

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Griffith Crack

mTm r

cT 2

c

EST e

T 2

Stress concentrations in cracks

From the collection of J.A. Franklin.

From the collection of J.A. Franklin.

Griffith Crack

31

31212cos

Griffith Failure Criterion

• Surface energy based derivation

• Not fit rock well

)(4 002 n

Empirical Failure Criterion

• Hoek and Brown single parameter empirical fit for sandstone

0.1331

ccc

m

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Fracture Propagation

• Griffith theory only deals with fracture initiation.

• No theoretical criteria for fracture propagation

• Photoelastic studies show mechanism

Fracture propagation

• Fracture can propagate along en echelon flaws (e.g. tension gashes)

Tension gashes Micro-seismics (Noise)

• Breaking rock makes noise

• Kaiser effect

From the collection of J.A. Franklin.

Time-strain curves Hydrofracturing

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Conjugate Fractures

• Angle of fracture = 45+ phi/2 from Sigma 1

• Dihedral Angles between conjugate joints = 60, 120, not 90, 90

Conjugate fractures and stresses

Typical Jointing

From the collection of J.A. Franklin. From the collection of J.A. Franklin.

Monroe and Wicander

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From the collection of J.A. Franklin. From the collection of J.A. Franklin.

Joint Origin Theories

• Tension– undulating

– irregular surface

– break along inclusions

– require a complex, perpetually shifting stress regime, exactly orthogonal to each other

• Shear– planar

– regular surface

– cut across inclusions

– should indicate shear movement

– conjugate angles should be about 60 degrees rather than 90

Plumatose structure

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From the collection of J.A. Franklin.

Origin of Joints (Hypothesis), after Price

• Tension

• Compressional shear

• Torsional/Fatigue mechanism

The Enigma of jointing, after Scheidegger

• Joints are ubiquitous

• Most joints are “ordinary” i.e. not special cases that are easily explained

• Ordinary joints are planar, relatively smooth, with no markings or signs of motion. Heterogeneities like inclusions are commonly cleanly cut but not displaced

Enigma of jointing (con’t)

• Typically 3 joint sets, one sub-horizontal, 2 near vertical, almost orthogonal

• Fresh non-lithological joints (tectonic joints) cut across obvious older joints

• Measurements several km’s apart are usually consistent with each other

Enigma of jointing (con’t)

• Outcrops within 10-20 km show similar joint orientations, but 1/4 of them show anomalous orientation, rotated up to 30 degrees

• One strong lithological maximum, 2 strong mutually near orthogonal tectonic maximum, and two weaker rotated tectonic maximums

• Principle horizontal (paleo) stress is bisectrix of tectonic maximums

From the collection of J.A. Franklin.

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Enigma of jointing (con’t)-Tension cracks

• Tensional stresses not common in nature

• Tensional stresses during cooling, but don’t result in orthogonal joints but hexagonal

• Effective tension stresses possible, result in hydrofracturing, but stress is quickly relieved by a single fracture

Enigma of jointing (con’t) -Pressure cracks

• Crack grows parallel to minimum principle stress, but this stress needs to be extremely small, or buckling will not take place

Enigma of jointing (con’t)-Slatey cleavage

• Ubiquitous

• Needs to be considered

• Easy to explain

From the collection of J.A. Franklin.

Enigma of jointing (con’t) -Shear fractures - Paradox 1

• Given that sigma 3 is vertical near the ground surface, expect “type B” shear joints by Mohr-Columb

• Get instead “type C”, near vertical joint proliferating

Enigma of jointing - Shear fractures - Paradox 2 - Dihedral Angle

From the collection of J.A. Franklin.

• Tectonic Concepts: Types of deformation

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Tectonic concepts: Crustal shortening

Joint persistence and termination

Model of Infinitely Persistent Joints

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Model of Penny-Shaped Cracks